Method of preparing an antibacterial polymer and its application

ABSTRACT

A method of preparing an antibacterial polymer and application of the antibacterial polymer obtained by this method. A silver oxide and a thermoplastic copolymer that contains a unit consisting of ethylenically unsaturated carboxylic acid or an anhydride thereof and that can be melted upon heating, are mixed, melted and is kneaded in order to obtain a polymer that exhibits in combination the antibacterial property and the heat-processability or the heat-moldability. By making this polymer present on the inner surface of the packing containers, it is made possible to impart the antibacterial property without deteriorating the flavor retentivity for the contents.

TECHNICAL FIELD

The present invention relates to a method preparing an antibacterialpolymer and its application, and particularly to an antibacterialpacking material using the same polymer.

BACKGROUND ART

It has long been known that compounds of silver exhibit antibacterialproperty. For instance, calcium carbonate to which silver is added hasbeen extensively used for the sterilization of the dechlorinated tapwater.

It has also been known that antibacterial property is exhibited by apolymer that contains silver acrylate and/or silver methacrylate as astructural unit. Japanese Patent Publication No. 43123/1983 discloses anantibacterial semi-permeable composed of the above polymer and acellulose ester.

Furthermore, Japanese Laid-Open Patent Publication No. 154746/1988discloses an antibacterial film composed of a synthetic resin whichcontains dispersed therein a hygroscopic agent and zeolite-type solidparticles that hold metal ions such as silver ions that exhibitantibacterial action by the exchange of ions, the zeolite-type solidparticles having a specific surface area of greater than 150 m² /g andan SiO₂ /Al₂ O₃ molar ratio of smaller than 14.

According to the method of producing the antibacterial polymer of theformer publication, the silver acrylate and/or the silver methacylate ispolymerized, or silver nitrate is reacted with polyacrylic acid,polymethacrylic acid or a copolymer thereof. However, the antibacterialpolymer that is obtained lacks thermal stability under dry heatedconditions, and does not apparently lend itself for such applications asforming films by the heat treatment or forming containers through theheat-molding.

According to the method of blending the silver ion-exchanged zeolite inthe resin disclosed in the latter publication, there remain problems inregard to producing the silver ion-exchanged zeolite and in regard tothat the sterilizing power decreases if there exist silver oxides in thezeolite, though there is problem with regard to heat resistance. Therestill exists a problem in that the silver ion-exchanged zeolite isalkaline just as the zeolite is in general, and hence tends to hydrolyzethe resin that is blended.

DISCLOSURE OF THE INVENTION

The present inventors have discovered the fact that a polymer havingantibacterial property is obtained when a silver oxide and athermoplastic copolymer that contains a unit consisting of ethylenicallyunsaturated carboxylic acid or an anhydride thereof and that can bemelted upon heating, are melted and kneaded together, and that thepolymer can be heat-treated or heat-molded making it possible to obtaina container having antibacterial property if the innermost layer of thecontainer is formed by using this polymer.

That is, the object of the present invention is to provide anantibacterial polymer that has a combination of excellent antibacterialproperty and heat-moldability or heat-processability, and that can beproduced cheaply and easily, as well as a method of preparing the samepolymer.

Another object of the present invention is to provide an antibacterialpacking material that is free from the above-mentioned defects inherentin the prior art, that exhibits excellent antibacterial action as wellas excellent flavor retentivity for the contents.

According to the present invention, there is provided a method ofpreparing an antibacterial polymer comprising mixing a silver oxide anda thermoplastic copolymer that contains a unit consisting ofethylenically unsaturated carboxylic acid or an anhydride thereof andthat can be melted upon heating, and then melting and kneading themixture.

According to the present invention, furthermore, there is provided anantibacterial packing material wherein at least the inner surface layerof the packing material contains an antibacterial polymer that a formedby kneading a silver oxide and a thermoplastic copolymer that contains aunit consisting of ethylenically unsaturated carboxylic acid or ananhydride thereof and that can be melting upon heating, theantibacterial polymer containing at least a moiety of the silver oxidein the form of a silver salt.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram showing FI-IR spectra of an ethylene-methacrylicacid copolymer containing 3% by weight of silver oxide (Example 14); and

FIG. 2 is a diagram showing FI-IR spectra of an ethylene-methacrylicacid copolymer (Comparative Example 4).

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention is based on the discovery that when a silver oxideand a thermoplastic copolymer that contains a unit consisting ofethylenically unsaturated carboxylic acid or an anhydride thereof andthat can be melted upon heating are melted and kneaded together, thesilver component is dispersed in, in comparison with the case whereother polymers or silver compounds are used, the polymer veryhomogeneously and finely, the silver oxide component is at least partlyinverted into a silver salt, and the obtained polymer exhibits excellentantibacterial property as well as resistance large enough to withstandthe heat treatment at the time of heat-molding and coating.

In the present invention, first, it is important to use a thermoplasticcopolymer that contains a unit (hereinafter referred to as acid unit)consisting of ethylenically unsaturated carboxylic acid or an anhydridethereof and that can be melted upon heating as a polymer, as well as touse a silver oxide as a silver component.

That is even when a polymer without containing the above acid unit ismelted and blended with the silver oxide, the blend exhibits almost noantibacterial property and, further, the silver oxide is poorlydispersed in the polymer. The acid unit-containing polymer which is acopolymer that can be melted upon heating plays an important role forbeing kneaded together with the silver oxide, and further playsimportant roles for imparting heat-processability at the time of coatingby which the blend can be used as a packing material and for impartingheat-moldability at the time of molding the packing material.

When other silver components such as silver acetate and the like areused, the antibacterial property is not exhibited to a satisfactorydegree unless they are used in considerably large amounts compared withwhen the silver oxide is used. In this case, furthermore, it isdifficult to homogeneously and finely disperse the silver component inthe polymer matrix.

When the copolymer containing the acid unit and the silver oxide aremelted and kneaded together according to the present invention, at leastmoiety of the silver oxide is coupled to the acid unit to form a silversalt. This fact is confirmed from the measurement that the carboxylgroup (--COOH) of acid unit in the copolymer is of the ionic type(--COO⁻). The accompanying FIG. 2 is a diagram showing infrared-rayabsorption spectra of a copolymer containing acid unit of a startingmaterial of Example 4 that will be described later, and FIG. 1 is adiagram showing infrared-ray absorption spectra of a copolymer that ismelted and kneaded together with the silver oxide. In the former case,the characteristic absorption of COOH is recognized near a wavenumber of1697 cm⁻¹ but no characteristic absorption of COO⁻ is recognized. In thelatter case, the characteristic absorption of COO⁻ appears at thewavenumbers 1530 to 1540 cm⁻¹ indicating the formation of a silver salt.

According to the present invention, the copolymer exhibits excellentantibacterial property being closely related to the fact that the silvercomponent exists in the form of a salt that is coupled to the acid unit,and that the polymer contains the acid unit to permit the permeation ofwater that activates the silver component. This is in good agreementwith the fact that almost no effect is exhibited by the polyethylene inwhich are blended silver oxide and silver acetate.

Besides, the copolymer that is kneaded together with the silver oxidedoes not give offensive taste or offensive odor to the packed content,but exhibits excellent flavor retentivity for the packed content andfurther exhibits antibacterial property.

According to the present invention, example of the acid unit of thecopolymer include ethylenically unsaturated carboxylic acids such asmethacrylic acid, acrylic acid, crotonic acid, itaconic acid, fumaricacid, maleic acid, anhydrous maleic acid and anhydrous itaconic acid, oranhydrides thereof, that are used alone or in a combination of two ormore kinds. From the standpoint of thermal stability, it is preferred touse the acid unit consisting chiefly of methacrylic acid.

From the standpoint of melting property and thermal stability, the aboveacid unit must be a copolymer with a monomer constituting a polymer thathas melt-moldability. The most preferred examples of such copolymers,are olefins such as ethylene, propylene, butene-1, pentene-1, andheptene-1. Among them, ethylene is preferred. Other copolymerizablemonomers include vinyl-type or acrylic-type monomers such as styrene,vinyl chloride, (meth)acrylic acid ester, vinyl acetate andacrylonitrile .

The content of acid unit in the copolymer should generally range from0.2 to 35 mole % and, preferably, from 2 to 25 mole % from thestandpoint of both the antibacterial property and the thermal stability.Further, the copolymer that is used should have a melting point (orsoftening point when the melting point is not obvious) of generally 70°to 220° C., and particularly, 80° to 210° C. Furthermore, the molecularweight of the copolymer should lie within a range in which the films canbe formed.

The copolymer which is particularly adapted to achieving the object ofthe present invention comprises ethylenically unsaturated carboxylicacid or an anhydride thereof in an amount of 0.2 to 35 mole % and,particularly, in an amount of 2 to 25 mole %, olefins in an amount of 65to 99.8 mole % and, particularly, in an amount of 75 to 98 mole %, and(meth)acrylic acid ester in an amount of 0 to 35 mole % and,particularly, in an amount of 0 to 25 mole %.

The silver oxide should have a purity of higher than 98 % and,particularly, higher than 99 %, and a particle size which generally isas fine as 10 μm or smaller, and particularly 5 μm or smaller.

The amount of blending the silver oxide with respect to the copolymershould generally be from 0.0005 to 10 % by weight and, particularly,from 0.001 to 5 % by weight from the standpoint of cost, antibacterialproperty and dispersibility, though the amount may vary depending uponthe applications and the required antibacterial property.

To prepare the antibacterial polymer, the copolymer and the silver oxideare pre-mixed by a method which is usually called dry blend. Thisenables the silver oxide to uniformly adhere on the surfaces of thecopolymer particles. The pre-mixing can be effected by using a ribbonblender, a conical blender, the Henschel's mixer, a ball mill, or asuper mixer.

The pre-mixture is then kneaded at a temperature which is higher thanthe melting temperature of the copolymer but is lower than thedecomposition temperature of the copolymer. Usually, the kneadingtemperature should range from 90° to 220° C., and the kneading ispreferably effected in an insert atmosphere such as in an nitrogenatmosphere using a monoaxial or biaxial extruder, a kneader, or theBumbery's mixer. According to the present invention, the feature residesin that the kneading operation is carried out at a relatively lowtemperature in a short period of time, and at least moiety of the silveroxide is inverted into a silver salt through the kneading operation.

It is preferred that the silver component in the form of a silver saltis contained at a concentration of greater than 5×10⁻³ milligram atomsand, particularly, 10⁻² to 50 milligram atoms per 100 g of thecopolymer.

The melted and kneaded product of the present invention is used forforming a packing material, by itself or in the form of a blend togetherwith other plastic material for forming containers, or in the form of acomposition together with a resin for forming coating or together with acoating material.

For instance, the above melted and kneaded product is subjected to theT-die method or the inflation film-forming method in order to obtain afilm for packing and a sheet for forming containers. Further, thekneaded product is extruded through a circular die to form a Parison forblow-molding. The sheet is subjected to the vacuum molding,compressed-air molding or plug-assist molding in order to form plasticcontainers, and the Parison is blow-molded to form plastic containershaving the shape of a bottle.

At the time of extrusion molding, a base resin layer such as ofpolyethylene, polypropylene or polyester and a layer of the kneadedcopolymer of the present invention are allowed to meet together in thedie, and are extruded together thereby to form a multi-layer film or amulti-layer plastic container.

Moreover, the antibacterial polymer film of the present invention thathas been formed in advance is subjected together with other resin filmor a metal foil or a combination thereof, to widely known means such asthe sandwich lamination, dry lamination or extrusion coating method, inorder to obtain a multi-layer film or a laminate for forming multi-layercontainers.

Furthermore, the melting and kneaded product is fed to a pelletizer topulverize it as required, or is dispersed in water to obtainantibacterial particles that will be used being dispersed in otherresins or the coating materials.

The particles of the antibacterial polymer are blended in a moldingpolymer such as polyethylene, and are subjected to the same means as theone described above, in order to form single-layer or multi-layerplastic containers. The antibacterial polymer of the present inventioncan be blended in other polymers for molding in an amount of 1 to 80 %by weight and, particularly, in an amount of 5 to 80 % by weight.

Furthermore, the powder of the antibacterial polymer can be used forcoating the inner surfaces of cans, caps, can lids and the like, beingdispersed in the coating materials that are known per se for costing theinner surfaces of packing materials, such as epoxyphenol-type coatingmaterial epoxyamino-type coating material, vinyl-type coating material,acrylic-type coating material, phenol-type coating material, alkid-typecoating material, and organosol-type coating material. In this case, theantibacterial polymer should have a particle size of from 0.1 to 10 μmand, particularly, from 0.1 to 5 μm, and should further be contained inan amount of 1 to 80 % by weight with respect to the solid component ofthe coating material.

The antibacterial polymer of the present invention can be contained inthe liners and packings of the type of olefin resin or of the type ofvarious elastomers fitted to the inner surfaces of the caps, or can becontained in the sealing gaskets and in the heat sealants for a varietyof containers.

EMBODIMENTS

Described below are the embodiments of the present invention in whichthe effects of sterilization are evaluated in compliance with thefollowing testing method.

Test for evaluating antibacterial property

A suspension of escherichia coli IAM1239 or staphylococcus aureusIAM12544 was so diluted that the test bacteria solution containedbacteria in a number of 2 to 4×10⁶ /ml. 0.1 Milliliter of the testbacteria solution was dropped on a sample cut in a size of 5×5 cm², andwas held under this condition maintained at 37° C. for 24 hours. After24 hours have passed, the sample was washed with 30 ml of physiologicalsaline solution, and the number of living bacteria in the washingsolution was measured by the membrane filter method. The culture wascarried out using a bouillon culture ground for general bacteria(produced by Nissui Seiyaku Co.) at a temperature of 37° C. for 24hours.

Resins used in the embodiments of the present invention are shown inTable 1 below.

                  TABLE 1                                                         ______________________________________                                        Brief       Resin monomer    Melting                                          Designation composition      point                                            ______________________________________                                        EO          100    % E                140° C.                          EMO1        99.9   % E/0.1  % MAA  130                                        EM2         98     % E/2    % MAA  110                                        EM9         91     % E/9    % MAA   96                                        EM15        85     % E/15   % MAA   85                                        EA9         91     % E/9    % AA   100                                        ______________________________________                                         E: ethylene                                                                   MAA: methacrylic acid                                                         AA: acrylic acid                                                         

ANTIBACTERIAL MATERIALS Examples 1, 2, 3 and 4

An ethylene-methacrylic acid copolymer EM9 was mixed with 0.003 to 3% byweight of a silver oxide using a mixer at a mixing temperature of 150°C. The mixture was then heated and pressed by a hot-press machine at200° C. for two minutes to obtain a film having a thickness of 50 μm.

Ethylene-methacrylic acid copolymers EM2, EM9, EM15 and ethylene-acrylicacid copolymer EA9 were, respectively, mixed with 0.05% by weight of thesilver oxide using the mixer at a mixing temperature of 150° C. Themixture were then heated and pressed by the hot-press machine at 200° C.for two minutes to obtain films all having of 50 μm.

COMPARATIVE EXAMPLES 1, 2, 3, 4, 5 and 6

The resins of Table 1 were heated and pressed by the hot-press machineat 200° C. for two minutes to obtain films all having a thickness of 50μm.

COMPARATIVE EXAMPLE 7

The ethylene-methacrylic acid copolymer EM9 was mixed with 0.0003 byweight of the silver oxide in the same manner as the above examples toobtain a film.

COMPARATIVE EXAMPLES 8, 9 and 10

The ethylene-methacrylic acid copolymer EM9 was mixed with 1% by weightof a silver powder (325 mesh), silver chloride or silver acetate using amixer at a mixing temperature of 150° C. The mixtures were then heatedand pressed using the hot-press machine at 200° C. for two minutes toobtain films all having a thickness of 50 μm.

Note) Comparative Example 10 exhibits antibacterial property emitting,however, the odor of acetic acid. The prepared film emits the odor ofacetic acid, too, lending itself not for being used as a packingmaterial.

EXAMPLES 5, 6, 7 and 8

The ethylene-methacrylic acid copolymer EM9 was mixed with 0.1% byweight of the silver oxide using the mixer at mixing temperatures of125° C., 150° C., 175° C. and 200° C. The mixture were heated andpressed by the hotpress machine at 200° C. for two minutes to obtainfilms all having a thickness of 50 μm.

COMPARATIVE EXAMPLE 11

The ethylene-methacrylic acid copolymer EM9 was mixed with 0.1% byweight of the silver oxide using the mixer at a mixing temperature of225° C. The mixture was heated and pressed by the hot-press machine at200° C. for two minutes to obtain a film having a thickness of 50 μm.

COMPARATIVE EXAMPLE 12

The polyethylene EO was mixed with 1% by weight of the silver oxideusing the mixer at a mixing temperature of 170° C. The mixture was thenheated and pressed by the hot-press machine at 200° C. for two minutesto obtain a film having a thickness of 50 μm.

COMPARATIVE EXAMPLE 13

The ethylene-methacrylic acid copolymer EM01 was mixed with 0.1% byweight of the silver oxide using the mixer at a mixing temperature of170° C. The mixture was then heated and pressed by the hot-press machineat 200° C. for two minutes to obtain a film having a thickness of 50 μm.

Table 2 shows the results of antibacterial test of the samples.

                                      TABLE 2                                     __________________________________________________________________________                            Antibacterial test                                                  Content   (Escherichia coli)                                                                             (Staphylococcus aureus)                            of silver                                                                           Mixing                                                                            Number of living Number of living                                   compound                                                                            temp.                                                                             bacteria/25 cm.sup.2                                                                           bacteria/25 cm.sup.2                 Resin     Kind                                                                              (wt %)                                                                              (°C.)                                                                      (Preserved of 37° C. for 24                                                             (Preserved of 37° C. for                                               24 hours)                            __________________________________________________________________________    Comp.                                                                         Example                                                                       1     EO   --           5.0 × 10.sup.5                                                                           3.7 × 10.sup.5                 2     EMO1                                                                               --           1.9 × 10.sup.5                                                                           2.5 × 10.sup.5                 3     EM2  --           4.1 × 10.sup.5                                                                           2.0 × 10.sup.5                 4     EM9  --           2.8 × 10.sup.5                                                                           4.0 × 10.sup.5                 5     EM15                                                                               --           5.7 × 10.sup.5                                                                           4.3 × 10.sup.5                 6     EM9  --           3.0 × 10.sup.5                                                                           4.5 × 10.sup.5                 Example                                                                       1     EM9 Ag.sub.2 O                                                                         0.003                                                                              150 8.0 × 10.sup.3                                                                           5.0 × 10.sup.3                 2              0.03 150 <30              <30                                  3              0.3  150 <30              <30                                  4              3.0  150 <30              <30                                  Comp.                                                                         Example                                                                       7     EM9 Ag.sub.2 O                                                                         0.0003                                                                             150 2.0 × 10.sup.5                                                                           4.0 × 10.sup.5                 8     EM9 Ag   1    150 4.2 × 10.sup.5                                                                           3.1 × 10.sup.5                 9         AgC1 1    150 3.7 × 10.sup.5                                                                           4.0 × 10.sup.5                 10        AgAc 1    150  30                <30 *)                             Example                                                                       5     EM9 Ag.sub.2 O                                                                         0.05 125 <30              <30                                  6         Ag.sub.2 O                                                                         0.05 150 <30              <30                                  7         Ag.sub.2 O                                                                         0.05 175 <30              <30                                  8         Ag.sub.2 O                                                                         0.05 200 7.1 × 10.sup.3                                                                           3.0 × 10.sup.3                 Comp.                                                                         Example                                                                       11    EM9 Ag.sub.2 O                                                                         0.05 225 4.5 × 10.sup.5                                                                           2.8 × 10.sup.5                 Example                                                                       9     EM2 Ag.sub.2 O                                                                         0.1  170 5.0 × 10.sup.3                                                                           1.8 ×  10.sup.3                10    EM9 Ag.sub.2 O                                                                         0.1  170 <30              <30                                  11    EM15                                                                              Ag.sub.2 O                                                                         0.1  170 <30              <30                                  12    EM9 Ag.sub.2 O                                                                         0.1  170 <30              <30                                  Comp.                                                                         Example                                                                       12    EO  Ag.sub.2 O                                                                         0.1  170 3.1 × 10.sup.5                                                                           4.3 × 10.sup.5                 13    EMO1                                                                              Ag.sub.2 O                                                                         0.1  170 4.4 × 10.sup.5                                                                           1.7 × 10.sup.5                 __________________________________________________________________________

BLEND

The ethylene-methacrylic acid copolymer EM9 was mixed with 0.3% byweight of the silver oxide using the mixer at a mixing temperature of150° C. to obtain a mixture which is hereinafter referred to as K1.

EXAMPLES 13, 14, 15 and 16

A polyethylene (density 0.92) (simply referred to as E1) was mixed withthe above mixture K1 in an amount of 5 to 50% by weight using the mixerat a mixing temperature of 170° C., and was heated and pressed by thehot-press machine at 200° C. for two minutes to obtain a film having athickness of 50 μm.

COMPARATIVE EXAMPLE 14

The polyethylene E1 was mixed with the above mixture K1 in an amount of1% by weight using the mixer at a mixing temperature of 170° C., and washeated and pressed by the hot-press machine at 200° C. for two minutesto obtain a film having a thickness of 50μum. Comparative Example 15,16, 17 and 18.

The polyethylene El was mixed with a resin EM3 in an amount of 5 to 50%by weight using the mixer at a mixing temperature of 170° C., and washeated and pressed by the hot-press machine at 200° C. for two minutesto obtain a film having a thickness of 50μm.

Table 3 shows the results of antibacterial test of the samples.

                                      TABLE 3                                     __________________________________________________________________________                          Antibacterial test                                                            (Escherichia coli)                                                                             (Staphylococcus aureus)                                  Mixing                                                                            Number of living Number of living                                         temp.                                                                             bacteria/25 cm.sup.2                                                                           bacteria/25 cm.sup.2                          Resin                                                                             (%) (%)                                                                              (°C.)                                                                      (Preserved at 37° C. for 24                                                             (Preserved at 37° C. for 24                                            hours)                                 __________________________________________________________________________    Example                                                                       13     KI   5/EI                                                                             95 170 3.7 × 10.sup.4                                                                           8.4 × 10.sup.4                   14         10  90     8.8 × 10.sup.2                                                                           1.8 × 10.sup.3                   15         30  70     <30              <30                                    16         50  50     <30              <30                                    Comparative                                                                   Example                                                                       14     KI   1/EI                                                                             99 170 2.5 × 10.sup.5                                                                           1.0 × 10.sup.5                   15     EM3  5/EI                                                                             95 170 2.8 × 10.sup.5                                                                           4.0 × 10.sup.5                   16         10  90     1.3 × 10.sup.5                                                                           3.7 × 10.sup.5                   17         30  70     4.2 × 10.sup.5                                                                           3.0 × 10.sup.5                   18         50  50     4.5 × 10.sup.5                                                                           2.9 × 10.sup.5                   __________________________________________________________________________

COATING MATERIAL

The powder used for the examples was prepared as described below.

The ethylene-methacrylic acid copolymer EM9 was admixed with 0.3% byweight of the silver oxide, extruded by a monoaxial extruder at 150° C.,and was pelletized. The pellets were pulverized into a powdery form. Thegranules having sizes of smaller than 10 μm were separated and used forthe experiment. The granules having sizes of smaller than 10 μm wereseparated and used for the experiment. The granules are hereinaftersimply referred to as PK1.

The pellets of ethylene-methacrylic acid copolymer EM9 were similarlypulverized and separated. The granules thereof are simply referred to asPEM. Examples 17, 18, 19 and 20.

In the epoxyphenol-type coating material was dispersed the above powderPK1 in an amount of 5 to 50% by weight with respect to the solidcontent. Then, the coating material was applied using a bar coater ontoa steel plate (0.18 mm thick) that has been electrolytically plated withchromium, and was heated and dry-baked in a gas oven at 200° C. for oneminute. The dry film possessed a thickness of about 20 μm.

COMPARATIVE EXAMPLE 19

The epoxyphenol-type coating material was applied using a bar coateronto the steel plate (0.18 mm thick) that has been electrolyticallyplated with chromium, and was heated and dry-baked in the gas oven at200° C. for one minute. The dry film possessed a thickness of about 29μm.

COMPARATIVE EXAMPLE 20

In the epoxyphenol-type coating material was dispersed the above powderPK1 in an amount of 1% by weight with respect to the solid content.Then, the coating material was applied using a bar coater onto the steelplate (0.18 mm thick) the has been electrolytically plated withchromium, and was heated and dry-baked in the gas oven at 200° C. forone minute. The dry film possessed a thickness of about 20 μm.

COMPARATIVE EXAMPLES 21, 22, 23, 24 and 25

In the epoxyphenol-type coating material was dispersed the above powderPEM in an amount of 1 to 50% by weight with respect to the solidcontent. Then, the coating material was applied using a bar coater ontothe steel plate (0.18 mm thick) that has been electrolytically platedwith chromium, and was heated and dry-baked in the gas oven at 200° C.for one minute. The dry film possessed a thickness of about 20 μm.

The bacterial tests were carried out in the same manner as the case ofthe films by cutting the coated plates into 5×5 cm². The results were asshown in Table 4.

                                      TABLE 4                                     __________________________________________________________________________    Paint: epoxyphenol                                                                                 Antibacterial test                                                            (Escherichia coli)                                                                             (Staphylococcus aureus)                                      Number of living Number of living                                             bacteria/25 cm.sup.2                                                                           bacteria/25 cm.sup.2                           Amount of powder blended                                                                    (Preserved at 37° C. for 24                                                             (Preserved at 37° C. for 24                                            hours)                                  __________________________________________________________________________    Example                                                                       17     PK1     5%    5.6 × 10.sup.4                                                                           8.1 × 10.sup.4                    18            15%    7.1 × 10.sup.2                                                                           1.2 × 10.sup.3                    19            25%    <30              <30                                     20            50%    <30              <30                                     Comparative                                                                   Example                                                                       19             1%    4.3 × 10.sup.5                                                                           3.9 × 10.sup.5                    20     PK1     1%    3.5 × 10.sup.5                                                                           6.1 × 10.sup.5                    21     PEM     1%    6.0 × 10.sup.5                                                                           5.1 × 10.sup.5                    22             5%    5.3 × 10.sup.5                                                                           4.8 × 10.sup.5                    23            15%    4.7 × 10.sup.5                                                                           3.3 × 10.sup.5                    24            25%    4.2 × 10.sup.5                                                                           6.9 × 10.sup.5                    25            50%    5.1 ×  10.sup.5                                                                          3.5 × 10.sup.5                    __________________________________________________________________________

EXAMPLE 21

The ethylene-methacrylic acid copolymer (methacrylic acid, 9 mole %) wasmixed with 0.05% by weight of the silver oxide, and was pelletized (EM9(0.05 Ag₂ O)) by using the monoaxial extruder. The pellets andpolypropylene were extruded together to form a multi-layer sheet. Themulti-layer sheet consisting of an inner layer EM9 (0.05 Ag₂ O) 10 μmthick and an outer layer PP 800 μm thick was molded into a round cup(having a flange diameter of 80 mm, an inner diameter of 70 mm and adepth of 35 mm) through the molten compressed-air molding.

The antibacterial test was carried out as described below.

A test bacteria solution of escherichia coli or staphylococcus sureusthat has been so diluted that the number of bacteria was 2 to 4×10⁶ /mlwas dropped in an amount of 0.1 ml on the inner surface of the cup andwas maintained under this condition at 37° C. for 24 hours. After 24hours have passed, the inner surface of the cup was washed with 30 ml ofphysiological saline solution, and the number of living bacteria in thewashing solution was measured by the membrane filter method. The culturewas carried out using a bouillon culture ground for general bacteria(produced by Nissui Seiyaku Co.) at a temperature of 37° C. for 24hours.

Table 5 shows the results of the antibacterial test.

                  TABLE 5*                                                        ______________________________________                                        Antibacterial test.                                                                        Number of living bacteria/cup                                                 (37° C. × 24 hours)                                 ______________________________________                                        Escherichia coli                                                                             30                                                             Staphylococcus aureus                                                                        30                                                             ______________________________________                                    

According to the present invention as is obvious from the foregoingembodiments, a thermoplastic copolymer the contains s unit consisting ofethylenically unsaturated carboxylic acid or an anhydride thereof andthat can be melted upon heating and a silver oxide are mixed togetherand are kneaded, in order to form a polymer that exhibits in combinationthe antibacterial property and the heat-processability or theheat-moldability. By making the polymer present on the surfaces of thecontainers for packing, it is made possible to impart the antibacterialproperty without deteriorating the flavor retentivity for the contents.The containers can be advantageously used as an aseptic packing route oras containers for packing fresh foods without requiring and particularsterilization operation.

We claim:
 1. A method of preparing an antibacterial polymer comprisingmixing a silver oxide and a thermoplastic copolymer that contains a unitconsisting of ethylenically unsaturated carboxylic acid or an anhydridethereof and that can be melted upon heating, and then melting andkneading the mixture.
 2. A method of preparing an antibacterial polymeraccording to claim 1, wherein said thermoplastic copolymer contains anethylenically unsaturated carboxylic acid or an anhydride thereof in anamount of 0.2 to 35 mole %, an olefin in an amount of 65 to 99.8 mole %,and an ethylenically unsaturated carboxylic acid ester in an amount of 0to 30 mole %.
 3. A method of preparing an antibacterial polymeraccording to claim 1, wherein the silver oxide is mixed in an amount of0.0005 to 10 % by weight with respect to said copolymer.
 4. Anantibacterial packing material wherein at least the inner surface layerof the packing material contains an antibacterial polymer that is formedby kneading a silver oxide and a thermoplastic copolymer that contains aunit consisting of an ethylenically unsaturated carboxylic acid or ananhydride thereof and that can be melted upon heating, the antibacterialpolymer containing at least a moiety of the silver oxide in the form ofa silver salt.
 5. A coating material to be applied to the inner surfaceof a packing material wherein said coating material contains dispersedtherein an antibacterial polymer that is formed by kneading a silveroxide and a thermoplastic copolymer that contains a unit consisting ofan ethylenically unsaturated carboxylic acid or an anhydride thereof andthat can be melted upon heating, the antibacterial polymer containing atleast a moiety of the silver oxide in the form of a silver salt.